Hydrocarbon, such as crude oil, for making various grades of fuels and oils, is produced by drilling wells into the earth, either on land or under sea. Crude oil contains many kinds of hydrocarbons, including paraffin wax.
Paraffin wax deposition is found practically whenever hydrocarbon (e.g., crude oil) is produced and transported. Paraffin wax deposition obstructs the flow of oil, lowering oil production and interfering with transportation.
Paraffin wax deposits are typically made up of linear, saturated hydrocarbon chains (typically C16 to C80) admixed to branched hydrocarbons, asphaltenes, water, and inorganic compounds such as sand, rust, iron sulfide, clays, etc. The deposit hardness depends chiefly on the oil amounts and mineral substances present in the mixture.
The phenomenon of the deposition or precipitation of solid paraffin wax constitutes an example of fluid/solid phase equilibrium, which is explained in the light of the principles of solution thermodynamics. At higher pressures and temperatures, the higher molecular weight hydrocarbon of paraffin wax is dissolved in lower molecular weight hydrocarbon, which functions as a solvent for the paraffin.
The paraffin wax deposition mechanism depends on pressure and temperature. Generally, lower pressures tend to increase the cloud point temperature. The cloud point temperature range of several waxy crude oils is such that the wax can precipitate even on the formation face as well as within the formation throughout the pressure reduction phenomenon which normally occurs during the useful life of the well. The lowering in production rate can be wrongly attributed to the reservoir depletion, while many times it is due to the formation permeability reduction and/or to the reduction of the diameter of the production string.
Hydrocarbon chains of different lengths are present in varying percentages as well as melting points, becoming insoluble at different pressure and temperature conditions. Generally, paraffin wax precipitation from hydrocarbon is gradual.
Paraffin wax is a dissolved component of relatively hot crude oil. For example, subterranean formations bearing hydrocarbon are usually under high pressure and at temperatures that are higher than 120° F. (49° C.). The formation temperatures of deep formations are usually much higher than 120° F. (49° C.). But paraffin wax tends to precipitate from crude oil as waxy solid deposits when the pressure and temperature is lowered as the hydrocarbon is produced and as the crude oil cools down.
After the hydrocarbon flows from a subterranean formation at relatively high temperature into the production tubulars of the wellbore, it is transported through the tubulars of the wellbore to the wellhead. Further, the hydrocarbon produced at the wellhead must be transported to a refinery to be processed and separated into various components, e.g., to make various grades of fuels and oils. The most common method of transporting hydrocarbon is through pipelines.
But as crude oil flows through the production tubulars from the hydrocarbon-bearing formation through the wellbore toward the surface, the surrounding temperature of the wellbore tends to decline toward the surface, allowing heat to dissipate from the fluid and causing the hydrocarbon fluid to begin to cool. Further, as the crude oil is moved through cross-country pipelines, it can loose heat energy to the ground or environment and cool to a temperature well below 120° F. (49° C.). Thus, the hot crude oil from a subterranean formation tends to be cooled as it flows through these conduits. As the temperature of the crude oil falls, paraffin wax in the crude oil tends to become a solid, waxy material that falls out of the crude oil and paraffin deposits accumulate on the inner walls of the production tubing and pipelines. This can be particularly problematic in subsea pipelines because the surrounding water on the seafloor is very cold, typically about 39° F. (4° C.).
To help prevent paraffin deposits, some cross-country pipelines are heated, which is very costly. However, this is not feasible for subsea pipelines, which are in direct contact with the surrounding cold seawater.
As the paraffin wax deposits build up on the inside wall of a conduit, the opening for fluid flow through the pipeline becomes smaller and smaller. Unless at least some of the buildup is removed from time to time, eventually the deposits can increase to the point where the conduit becomes choked. Also, sometimes some of the paraffin deposits will release from the inside wall of a pipeline and cause a blockage. Such a blockage can occur anywhere in the pipeline. This paraffin deposition leads to reduced crude oil flow and under extreme conditions leads to complete blockage of the pipelines, as illustrated in FIG. 1.
Removal of the paraffin wax deposits is attempted through three main approaches: mechanical, thermal, and chemical. Often, a combination of two or more of these types of approaches is employed.
The mechanical approaches are used specially for production strings and pipeline conduits, as they are generally unsuitable for use in a subterranean formation or at the interface between a formation and a wellbore. In the mechanical methods, paraffin deposits are physically removed from the wall surface. Chemical surface agents and solvents can aid in the removal so that the dislodged paraffin deposits are dispersed in solution and can flow through the conduit.
The main mechanical approach for helping to clean a pipeline, including for removing paraffin buildup, is to run a mechanical device through the pipeline that scrapes the inner wall of the pipeline and pushes the paraffin through. This type of mechanical cleaning device is called a “pig” because the scraping of the inner wall of the pipeline makes a tremendous squealing noise. The pig is normally driven through the pipeline by a relatively high pumping pressure behind the pig, which is used to force the pig through the pipeline. But if paraffin buildup on the inner walls of the pipeline is heavy, as the pig moves forward it will collect so much paraffin in front of the moving pip that it blocks any further movement of the pig. In other words, as more and more paraffin is scraped off the inner walls of the pipeline and accumulates in front of the moving pig, at some point the pumping pressure in the pipeline is not enough to push the pig and all the accumulated paraffin further through the pipeline, at which point the pipeline is plugged.
The most common thermal method for removing paraffin deposits in production tubulars employs heated oil, so the technique is commonly referred to as “hot oiling.” The heated oil is usually pumped through the annulus between the wellbore and a production tubular, and then back to the surface through the production tubular in order to remove the paraffin wax deposits in the production tubular. This type of approach can be safely used only for production tubulars for paraffin wax deposits above the wellbore perforations to a hydrocarbon-bearing formation. Otherwise, the heated crude oil could carry some of the melted paraffin wax into the formation where it could cool and settle in the porous spaces, plugging the formation. Techniques using heated oil can also be used in pipelines to help remove paraffin wax deposits from the conduit.
One alternative to using heated oil is to use heated water. Water has a higher thermal capacity than oil, which capacity can be used to carry more heat energy to melt paraffin wax. Thus, the paraffin wax deposits can be thermally dispersed by hot water. However, hot water does not dissolve or dilute the paraffin wax, so the water and melted paraffin wax can form an undesirable oil/water emulsion. Another disadvantage to using hot water, however, is that it can contribute to corrosion problems with the metal conduits.
In U.S. Pat. No. 3,437,146 issued Apr. 8, 1969 to Clifford R. Everhart and Audra B. Cary, the abstract describes the disclosure as a method of removing paraffin deposits from a producing well including injecting heated xylene bottoms solvent into the wellbore at a temperature sufficiently high so that the solvent is at least 150° F. (65° C.) when it encounters the producing formation, and thereafter, withdrawing the solvent and dissolved paraffin.
Organic solvents like hot xylene are able to dissolve paraffins and asphaltenes, but not the inorganic materials in the paraffin wax deposits.
Numerous attempts have been made to remediate paraffin wax buildup in hydrocarbon-bearing subterranean formations and in conduits for the production or transmission of hydrocarbon.
In U.S. Pat. No. 4,178,993 issued Dec. 18, 1979 to Edwin A. Richardson and Ronald F. Scheuerman, the abstract describes the disclosure as a method of initiating production from a gas well which is kept from producing by the hydrostatic pressure of the liquid it contains, by injecting an aqueous liquid that contains reactants which form nitrogen gas within the well or reservoir and displaces enough liquid out of the well to lower the hydrostatic pressure to less than the fluid pressure in the adjacent portion of the reservoir and cause fluid to flow from the reservoir to the well. Further, this patent discloses that the nitrogen-gas-forming mixture can be an aqueous solution of ammonium chloride and sodium nitrite; or an aqueous solution of urea and sodium hypochlorite; or an aqueous solution of urea and sodium nitrite.
In U.S. Pat. No. 4,219,083 issued Aug. 26, 1980 to Edwin A. Richardson; Ronald F. Scheuerman; and David C. Berkshire, the abstract describes the disclosure as a backsurge of fluid through perforations in a well casing that is chemically induced by injecting into the surrounding reservoir a solution which contains (a) nitrogen gas-generating reactants, (b) a reaction-retarding alkaline buffer, and (c) a pH-reducing reactant that is capable of subsequently overriding the buffer, so that a rapid production of gas and heat causes a backsurging of fluid into the wellbore. Further, this patent further discloses that the nitrogen gas-generating reactants can be a nitrogen-containing compound and oxidizing agent comprising water-soluble salts of, respectively, ammonium hydroxide and nitrous acid.
In U.S. Pat. No. 4,330,037 issued Mar. 18, 1982 to Edwin A. Richardson and Walter B. Fair, Jr., the abstract describes that a portion of a subterranean oil and water-containing reservoir is concurrently chemically heated and selectively increased in its effective permeability to oil by injecting a solution of compounds containing ammonium ions and nitrite ions, which react exothermically to generate gaseous nitrogen, and a reaction-rate-controlling buffer. The reactant concentration and the rate at which the solution is injected are arranged to provide a selected temperature increase within a selected portion of the reservoir.
In U.S. Pat. No. 4,399,868 issued Aug. 23, 1983 to Edwin A. Richardson and Walter B. Fair, Jr. the abstract describes the disclosure as fluid passageways between a well borehole and a subterranean reservoir which are both plugged and submerged within relatively dense brine that can be contacted with heat and oil solvent by arranging a nitrogen gas generating aqueous solution to be both reactive at the reservoir temperature and denser than the brine in the borehole and flowing alternating slugs of it and a liquid oil solvent into the zone to be treated. Further, this patent discloses that the solid materials that plug such fluid passageways are usually heat-sensitive and oil-solvent-soluble materials, such as paraffinic and/or asphaltenic solids. The fluid contains enough total dissolved solids to provide a solution density exceeding that of the brine in the borehole, such that a significant portion of the heating solution sinks into the column of brine in the borehole and reacts to yield heat and gas that contact the plugged fluid passageways.
U.S. Pat. No. 4,380,268 issued Apr. 19, 1983 to Keith R. Martin, the abstract describes the disclosure as recovery of gas and oil being enhanced by the removal of paraffin and other hydrocarbons from wells by flushing the wells with water containing a polymer of a primary alcohol and ethylene oxide plus sodium silicate. Further, the patent describes that the detergent degreaser comprises a polymer of a straight chain linear carbon alcohol that is ethoxylated with ethylene oxide.
In U.S. Pat. No. 4,755,230 issued Jul. 5, 1988 to Jefferson P. Ashton; Hal W. McSpadden; Tara T. Velasco; Hang T. Nguyen, the abstract describes the disclosure as a method for removing paraffin deposits from the interior of a hydrocarbon transmission conduit, such as a subsea pipeline. The method comprises the steps of introducing into an isolated length of the conduit containing the paraffin a pre-determinable amount of an emulsified mixture of an aqueous solution and a hydrocarbon solution. The composition used in the method incorporates an aqueous solution which comprises in-situ nitrogen-generating components together with a sufficient amount of a buffered pH adjuster to produce a buffered pH value for the aqueous solution to abate the time of the reaction rate of the nitrogen-generating components to a level permitting introduction of the components into the isolated length prior to completion of any significant portion of the reaction required to effect temperature melting of the paraffin deposits. In a preferred form, a crystalline modifier may be incorporated into the hydrocarbon solution. After treatment, the solutions containing the dissolved paraffin are removed from the isolated conduit section. Further, the patent discloses that wherein the nitrogen-generating components comprise sodium nitrite in a concentration ranging from between about 3 to about 10 molar; and ammonium nitrate in a mole concentration approximately equal to that selected for the sodium nitrite.
A paper by J. P. Ashton et al., entitled “In Situ Heat System Stimulates Paraffinic Crude Producers in Gulf of Mexico,” in SPE Production Engineering, May 1989, p. 157-160, describes the thermal stimulation of wells in order to remove the paraffinic damage through the heat generation caused by an exothermic chemical reaction, in aqueous phase, the temperature of the formed brine reaching up to 248° F. (120° C.). The reaction rate is controlled to generate pre-determined amounts of heat at a previously established well depth. The injection of hot brine in the producing formation creates a heated region around the well perforations. The radial extension of the heated region is a function of the injected heated brine volume. As heat is transferred through vertical conduction through the perforated interval, formation areas of low permeability are equally heated. The exothermic reaction employs sodium nitrite and ammonium nitrate in aqueous solution, the reaction products being nitrogen, water and sodium nitrate. The resulting brine is not considered to be deleterious to the formation. The reaction occurs as soon as the forming salts are mixed, in the presence of HCl as catalyst, the control of the reaction being done by buffering the pH of the solution in the range of from 5.0 to 8.0. The reaction is faster at a lower pH. Control is effected such that the reaction begins gradually and progresses slowly as the solution is displaced throughout the production string at constant rate. Nearly 61 meters above the perforations, the reaction rate increases and produces huge amounts of heat, the temperature reaching a thermal maximum, heat being lost to the environment, with consequent reduction in the temperature of the spent solution.
In U.S. Pat. No. 5,183,581 issued Feb. 2, 1993 to Carlos N. Khalil; Regis K. Romeu; Andre Rabinovitz, the abstract describes the disclosure as a process, based on the Nitrogen Generating System/Emulsion in the presence of organic solvents, which is useful for the dewaxing of producing formations. The heat generation with the nitrogen reaction system and organic solvents gives rise to a thermo-chemical, synergistic system for long lasting removal of paraffinic damage, oil production rates being restored and even increased. The patent more particularly describes a process for the dewaxing of producing formations by means of a water-in-oil nitrogen generating emulsion system, which comprises the steps of: (a) preparing an aqueous solution of NH4Cl having a concentration of from 4.0 to 6.0M; (b) preparing an aqueous solution of NaNO2 having a concentration of from 5.0 to 9.0M; (c) preparing an organic solvent mixture to achieve the hot dissolution of the paraffinic damage; (d) adding an emulsifier to the organic solvents mixture so that the concentration of emulsifier in the mixture comprises between 0.5 to 2.0%; (e) adding acetic acid 96% to the NH4Cl solution; (f) adding 50% of the emulsified organic solvents mixture of step (d) to the NH4Cl and 50% of the mixture to the NaNO2 solution obtaining thus a NH4Cl emulsion and a NaNO2 emulsion respectively, both emulsions being kept under agitation; (g) pumping simultaneously into the well the NH4Cl and NaNO2 emulsions, forming a mixture of equimolar NH4Cl and NaNO2 amounts thus initiating an equimolar reaction between the components mixture, this mixture producing nitrogen and heat, while pH is kept between 4.5 and 5.8; and (h) after the pumping of the treating fluid, displacing it from the well by means of an overflush with an aqueous saline fluid.
In U.S. Pat. No. 5,484,488 issued Jan. 16, 1996 to Paul R. Hart and Michael J. Brown, the abstract describes the disclosure as methods for removing paraffin wax deposits from the surfaces of oilfield production equipment during oil production by melting and subsequently dispersing the deposits. These methods utilize an acid compound and a neutralizer compound which react exothermally to melt the deposit and form a dispersant to remove the melted fragments of the deposit.
In U.S. Pat. No. 5,639,313 issued Jun. 17, 1997 to Carlos Nagib Khalil, the abstract describes the disclosure as a process for the thermo-chemical dewaxing of a hydrocarbon transmission conduit, which comprises, after assessment of the conduit internal effective volume, treating the wax-containing conduit with a water-in-oil emulsion, co-currently to the production flow. The emulsion contains inorganic reactants which generate nitrogen and heat, which fluidize the paraffin deposit which is later driven off by cleaning beds. The amount of removed paraffin is known by assessing the final internal effective volume.
In a Ph.D. dissertation entitled Fused Chemical Reactions to Remediate Paraffin Plugging in Sub-Sea Pipelines by Duc Anh Nguyen, having for Advisor H. Scott Fogler, published in August 2004 by the University of Michigan, ISBN 0-496-69361-2, an abstract describes the work as a timed release scheme of the citric acid-catalyst was used to fuse the highly exothermic reaction between ammonium chloride and sodium nitrite in a fused chemical reaction (FCR) system. The timed release was obtained by encapsulating the acid-catalyst in gelatin capsules then coating the capsules with a water-soluble polymer. The highly exothermic FCR system is demonstrated to be a feasible solution for the billion-dollar problem of paraffin deposition during crude oil production and transportation operations in sub-sea pipelines. Studies of the exothermic reaction in an isothermal reactor showed that hydrogen ions catalyze the reaction by changing the concentration of the reacting species, not by changing the reaction pathways. The rate-limiting step involves the SN2 reaction between aqueous molecular ammonia and nitrogen trioxide to form nitrosamine. The activation energy of the reaction was found to be about 65.7 kJ/mol experimentally, which compares very favorably (within 11%) with the value found from molecular modeling. The facilitated diffusion of hydrogen ions away from the polymer interface is the principal process that determines the rate limiting step as well as the overall rate of the polymer dissolution. The facilitated diffusion effect increases significantly with an initial increase in the carrier concentration, then approaches a limit at high carrier concentration. However, there are optimum values of the carrier's pKa and of the solution pH which give a maximum facilitation effect. A homogenous chemico-diffusion model can predict concentration profiles of all species across the diffusion boundary layer and polymer dissolution rates which agree well with experimental results. Good agreement between simulation and experimental results for the FCR system in both batch and flow conditions was achieved. Batch experiments showed that the heat release is controlled by the thickness of the polymeric coating and can be delayed as long as 20 hours. Flow experiments demonstrated that the FCR system could be controlled to provide a substantial amount of effective heat in-situ. This large amount of effective heat is sufficient to soften and melt the wax deposit. The model for the FCR system in flow conditions was also extended to apply in a typical sub-sea pipeline.
In U.S. Pat. No. 6,939,082 issued Sep. 6, 2005 to Benton F. Baugh, the abstract describes the disclosure as a method of taking a remotely operated vehicle to the ocean floor to land on and move along a subsea pipeline above or below the seafloor and repeatedly circulate seawater which has been heated electrically, mechanically, or chemically across the outer surface of the pipeline to melt hydrates or paraffins which have formed on the inside of the pipeline.
In European Application No. EP98300454 published Jan. 4, 2006 having for named inventors Celso Rodrigo De Souza and Carlos Nagib Khalil, filed Jan. 22, 1998 and entitled “Improved Method for the Thermo-Chemical Dewaxing of Large Dimension Lines, the independent claim is for a method for the thermo-chemical dewaxing of a hydrocarbon transmission conduit containing paraffin deposit, said method comprising the steps of: [sic] (b) maintaining said emulsion in said conduit under conditions sufficient to fluidize the paraffin deposit and to generate nitrogen gas and heat from the reaction of said oxidizing nitrogen salt and said reducing nitrogen salt; and (c) removing the fluidized paraffin deposit from said conduit, wherein the delayed action activator is a linear, aliphatic copolyanhydride solubilized in a polar organic solvent.
As demonstrated by these published efforts to remediate paraffin wax deposits in hydrocarbon-bearing formations or in conduits for the production or transmission of hydrocarbon, there continues to be a long-felt need to find a solution for this problem.